Power supply with low los making current limitation

ABSTRACT

The invention relates to a power supply comprising a storage capacitor (C) which can be charged with a rectified input current (U g ) by means of a current-limiting element (SBG,T l ) which is controlled by a drive circuit. The direct-axis component of current (I 1 ) and the direct-axis component of voltage (U l ) on the current limiting element (T l ) are supplied to the control circuit (AST) as input values and the current limiting element is controlled in such a way that it is not subjected to more than a predetermined maximum power loss (P max ).

[0001] The invention relates to a power supply having a rectifier for rectifying an input AC voltage and having an energy-storage capacitor which can be charged from the rectified input voltage via a current limiting element which is controlled by a drive circuit.

[0002] In appliances such as these, special precautions have to be taken in order to minimize the loads on the mains power supply system caused when these appliances are switched on.

[0003] For this purpose, a known power supply having inrush current limiting SBG and having a voltage regulator SRE is illustrated in FIG. 1. An input AC voltage U_(w) is in this case supplied, after rectification in a rectifier GLB, as a pulsed DC voltage U_(g) via a current limiting circuit SBG to an energy-storage capacitor C, the voltage across which is U_(c), and which is connected in parallel with the input of a voltage regulator SRE or some other load. U_(a) denotes the output voltage of the regulator SRE.

[0004] The energy-storage capacitor C is used for bridging short-term failures of the input voltage U_(g), which is in the form of a DC voltage or, in particular, an unsmoothed voltage which is produced at the output of a rectifier GLB. The energy-storage capacitor C generally has a high capacitance and, when the power supply is switched on, it must first of all be charged up, with the current limiting circuit SBG preventing excessively high inrush current surge values during this charging process.

[0005]FIG. 2 shows a further known power supply with inrush current limiting, in which the load current drawn from regulator SRE in this case does not flow via the current limiter SBG, in order to reduce the power loss. For the purposes of this application, the differences between FIG. 1 and FIG. 2 are, however, irrelevant.

[0006] As soon as the capacitor C has been charged up, the current limiting must assume a low resistance in order that the currents flowing do not cause any further losses and, if necessary, the capacitor C can emit its energy with a low internal impedance to the load. Such active current limiting is designed in a characteristic manner, for example with current sources.

[0007] When a capacitor is charged up via such current limiting, virtually the same amount of energy which is absorbed by the capacitor is converted into heat in the transistor which is used for current limiting. The power loss in the current limiting element is directly proportional to the voltage difference between the input voltage and the capacitor voltage, assuming that the limiting current is set such that it is constant. The power loss in the limiting element thus fluctuates between zero and a maximum value within one half-cycle of the input voltage U_(g). The power loss which occurs is particularly critical in power supplies which have to emit a relatively high current during normal operation since, as is self-evident, the limiting current must be higher than the maximum operating current. Owing to the short-term load on the limiting transistor which is normally used, this has had to be greatly derated, since it is necessary to design it for the pulse power loss at the voltage maximum in conjunction with the transient thermal impedance.

[0008] One object of the invention is thus to specify a power supply with a current limiting circuit, which allows the use of low-cost limiting elements with a low permissible power loss.

[0009] This object is achieved by a power supply of the type mentioned initially in which, according to the invention, the drive circuit is supplied as input variables with the series current and with the series voltage across the current limiting element, and the drive circuit is set up to drive the current limiting element such that it is loaded with no more than a maximum power loss which can be predetermined.

[0010] Thanks to the invention, which takes account not only of the current but also of the power, low-cost limiting elements can be used with the mains power supply system being effectively protected and, if necessary, the load as well.

[0011] In this case, it is possible in particular to provide that the drive circuit is set up to switch off the current via the current limiting element on reaching a limit voltage which corresponds to the maximum power loss.

[0012] Approximate consideration of the maximum power loss, which is nevertheless sufficient in most cases, can be achieved if the drive circuit is set up to regulate the current via the limiting element along a straight line which runs essentially below the hyperbola of the maximum power loss in the I/U family of characteristics.

[0013] On the other hand, with somewhat greater circuit complexity than the best possible solution, it is also possible to provide for the drive circuit to be set up via the limiting element to regulate the current essentially along a hyperbola which corresponds to the maximum power loss.

[0014] One practical implementation is distinguished in that the current limiting element is a series transistor connected in series with a measurement resistor, the control electrode of which series transistor is driven firstly via a further transistor in order to provide a current source and secondly via a comparator signal which is obtained by comparison of the voltage across the series transistor with the limit voltage.

[0015] In another practical implementation, the invention provides that the current limiting element is a series transistor connected in series with a measurement resistor and driven by a transistor whose input circuit is supplied firstly with a signal which is proportional to the voltage drop across the measurement resistor and secondly with a signal which is proportional to the voltage across the series transistor.

[0016] In one optimum implementation, the invention provides that the current limiting element is a series transistor connected in series with a measurement resistor and driven by two control amplifiers, with one control amplifier being supplied firstly with a reference value which corresponds to the maximum power loss and secondly with the output of a multiplier, which multiplies the measured series current by the voltage across the series transistor, and with the other control amplifier being supplied with the measured series current and with a reference value which corresponds to the maximum current.

[0017] The invention together with further advantages will be explained in more detail in the following text with reference to the attached drawings, in which:

[0018]FIG. 1 and FIG. 2 show two embodiments of known power supplies, corresponding to the prior art,

[0019]FIG. 3 shows a general embodiment of a power supply according to the invention, in the form of a block diagram,

[0020]FIG. 4 shows a circuit detail of a first embodiment of the invention,

[0021]FIG. 5 shows a circuit detail of a second embodiment of the invention, and

[0022]FIG. 6 shows a circuit detail of a third embodiment of the invention, and

[0023]FIG. 7 uses a U/I diagram to show possible control and switching-off characteristics of current limiting circuits according to the invention.

[0024] In the circuit according to the invention as shown in FIG. 3, a DC voltage U_(g) is likewise produced from an AC voltage U_(w) by means of a rectifier GLB. This voltage U_(g) is supplied via a current limiting element, such as a field-effect transistor T_(L) in this case, to the energy-storage capacitor C.

[0025] The series current i_(L) is measured by means of a current sensor SI, and a voltage sensor SU is used for measuring the voltage U_(L) across the transistor T_(L). The signals from the current and voltage sensors are linked to one another and are converted to a drive signal for the transistor T_(L) such that one of the U-I characteristics illustrated in FIG. 7 is produced across the transistor T_(L).

[0026]FIG. 4 shows a variant in which a field-effect transistor T_(L) is likewise used as the current limiting element. A very low-resistance measurement resistor Rm which is connected in series with the transistor T_(L) is used as the current sensor, with the voltage which is dropped across this measurement resistor Rm and is proportional to the series current i_(L) controlling a transistor T_(v) via its base-emitter junction. The collector circuit of this transistor T_(v) in turn drives the field-effect transistor T_(L). On the other hand, the control electrode of the field-effect transistor T_(L) also receives—via a resistor R_(v)—a switching-off signal from the output of a comparator K, one of whose inputs is supplied with the series voltage U_(L) across the transistor T_(L) and whose other input is supplied with a limit voltage U_(gr) as a reference voltage. In this context, reference should also be made to FIG. 7, which clearly shows that, on switching on in the circuit as shown in FIG. 4, current limiting first of all takes place along a horizontal straight line where I=I_(max) and, on reaching the limit voltage U_(gr), where I_(max)·U_(gr)=P_(max), switching-off takes place de facto along a steeply falling straight line.

[0027] In the embodiment of the invention shown in FIG. 5, in which the same reference symbols are used where they correspond to those in FIG. 4, the series voltage U_(L) is applied in the form of a corresponding current via a resistor R_(B) to the base of the drive transistor T_(v) with yet another resistor R_(S) being connected between the base of T_(v) and the junction point between the transistor T_(L) and the measurement resistor R_(m). In this variant, the current i_(L) is limited along a straight line (see FIG. 7) which is located underneath the hyperbola of the maximum power P_(max), or touches this hyperbola at one point.

[0028] While the embodiments based on FIG. 4 and FIG. 5 make use of the maximum power at only one point in each case, FIG. 6 shows a virtually ideal solution, which always operates with the maximum power below the predetermined maximum current. As in the first two embodiments, the current is measured using a measurement resistor R_(m) and is in this case limited to the value I_(max) by means of an operational amplifier N_(I). A multiplier MUL uses the current and the voltage across the transistor T_(L) to determine the instantaneous power loss, while a further operational amplifier N_(P) provides regulation at the predetermined reference value P_(max). The two operational amplifiers are connected to one another such that, for example by means of open collector outputs, the respectively lower output voltage is applied to the control electrode of the transistor T_(L). 

1. A power supply having a rectifier (GLB) for rectifying an input AC voltage (U_(w)) and having an energy-storage capacitor (C) which can be charged from the rectified input voltage (U_(g)) via a current limiting element (SBG, T_(L)) which is controlled by a drive circuit, characterized in that the drive circuit (AST) is supplied as input variables with the series current (i_(L)) and with the series voltage (U_(L)) across the current limiting element (T_(L)), and the drive circuit is set up to drive the current limiting element such that it is loaded with no more than a maximum power loss (P_(max)) which can be predetermined.
 2. The power supply as claimed in claim 1, characterized in that the drive circuit (K, T_(v)) is set up to switch off the current via the current limiting element (T_(L)) on reaching a limit voltage (U_(gr)) which corresponds to the maximum power loss.
 3. The power supply as claimed in claim 1, characterized in that the drive circuit (T_(v); MUL, N_(P), N_(I)) is set up to regulate the current via the limiting element (T_(L)) along a straight line which runs essentially below the hyperbola of the maximum power loss in the I/U family of characteristics.
 4. The power supply as claimed in claim 1, characterized in that the drive circuit is set up via the limiting element (T_(L)) to regulate the current (i_(L)) essentially along a hyperbola which corresponds to the maximum power loss (P_(max)).
 5. The power supply as claimed in claim 2, characterized in that the current limiting element is a series transistor (T_(L)) connected in series with a measurement resistor (R_(m)), the control electrode of which series transistor is driven firstly via a further transistor (T_(V)) and secondly via a comparator signal which is obtained by comparison of the voltage across the series transistor with the limit voltage (U_(gr)).
 6. The power supply as claimed in claim 4, characterized in that the current limiting element is a series transistor (T_(L)) connected in series with a measurement resistor (R_(m)) and driven by a transistor (T_(V)) whose input circuit is supplied firstly with a signal which is proportional to the voltage drop across the measurement resistor and secondly with a signal which is proportional to the voltage (U_(L)) across the series transistor (T_(L)).
 7. The power supply as claimed in claim 4, characterized in that the current limiting element is a series transistor (TL) connected in series with a measurement resistor (R_(m)) and driven by two control amplifiers (N_(P), N_(I)), with one control amplifier (N_(P)) being supplied firstly with a reference value which corresponds to the maximum power loss (P_(max)) and secondly with the output of a multiplier (MUL), which multiplies the measured series current (i_(L)) by the voltage (U_(L)) across the series transistor, and with the other control amplifier (N_(I)) being supplied with the measured series current and with a reference value which corresponds to the maximum current (i_(max)). 